This invention relates to improved processes for preparing alkali metal dispersions.
An alkali metal dispersion may be employed, for example, as (1) an additive to a crude diolefinic monomer to purify it through removal from the system of the active hydride which forms by reaction with the alkali metal; (2) one of the essential materials for the preparation of an alfin catalyst; (3) a catalyst for the polymerization of dienic or monoolefinic monomers; and (4) a reducing agent for the preparation of higher alcohols from fatty acids or derivatives thereof. In addition, they are utilized for other chemical reactions.
Known processes for the preparation of such dispersions include those which employ high speed agitation, a colloid mill, a jet stream, or ultrasonic waves. According to those prior art processes, alkali metal dispersions are obtained by initially finely dividing an alkali metal by physical means, while heating the metal at a temperature above its melting point, and thereafter dispersing the minute particles thus formed in a dispersion medium, and finally cooling the product below the melting point of the alkali metal while maintaining it in a dispersed state.
Efforts are made to obtain particles of small uniform size, it being recognized that the larger the surface area, the higher the reactivity of the metal. Normally the particles in alkali metal dispersions range in particle size from 1 to 100.mu., although it is preferred that the size does not exceed 20.mu.. This fineness, when combined with good dispersion stability, will provide ideal dispersions.
Batch and continuous procedures are utilized in the manufacture of alkali metal dispersions. The latter method is preferred for production on an industrial scale since the batch operation requires larger dispersion and cooling equipment and the quality of the resulting dispersion varies amongst different batches.
Techniques for continuously preparing alkali metal dispersions at temperatures above the melting points of the particular metals have been described, for example by I. Katt and M. Tashima: "Alkali Metal Dispersions, " D. Van Nostrand Co., 1961, pp. 51-55, and in Japanese Patent Publication No. 16923/66.
The article cited above describes a typical process for continuously preparing alkali metal dispersions at temperatures above the melting point of the metals. The process necessitates extended retention times, at least about 30 minutes on the average, to reduce the size of particles. Therefore, large dispersion equipment is required and the production rate is low. Another disadvantage is that large particles are mixed in the resulting alkali metal dispersion because of operating difficulties which are inherent in the process.
A method of overcoming this latter difficulty is proposed in the cited Japanese Patent Publication. The method seeks to avoid the difficulties mentioned above, but, unfortunately, requires even larger and more complex equipment to obtain a reasonable production rate.